When using FIFO Generator Core, the allowed data count width is less than it should be.
If you select the FIFO depth to be 16, you will need 5-bits to represent the full span of data count. However, the allowed data count width is only 1 to 4. You are not able to select 5 for the data count width.
If you concatenate FULL flag with DATA_COUNT, you will get a counter that spans the entire range that you need. The DATA_COUNT bits will count the number of words in the FIFO. However, when it is FULL or EMPTY, the DATA_COUNT value will be all 0s. Therefore, you need a way to differentiate between FULL and EMPTY. You can use the FULL flag for this purpose.